We investigate the aggregation behaviour of the donor-acceptor molecules p-DTS(FBTTh2)2 (“T1”) and p SIDT(FBTTh2)2 (“H1”) in MTHF solutions. Using optical spectroscopy, we find that T1 forms aggregates in solution while H1 aggregates only when processed as a thin film, but not in solution. Free energy molecular dynamics (MD) simulations based on force-fields derived from quantum-mechanical density functional theory fully reproduce this difference. Our simulations reveal that this difference is not due to the lengthy carbon side chains. Rather, the molecular symmetry of T1 allows for an aggregated state in which the central donor units are spatially well separated while a similar configuration is sterically impossible for H1. As a consequence, any aggregation of H1 necessarily involves aggregation of the central donors which requires, as a first step, stripping the central donor of its protective MTHF solvation shell. This unfavourable process leads to a significant kinetic hindrance for aggregation and explain the strongly differing aggregation behaviour of T1/H1 in MTHF despite their otherwise similar structure. By comparison to further donor-acceptor molecules with structure and symmetry similar to T1 and H1, referred to as CT and TT, respectively, we demonstrate that this aggregation behaviour results from the individual building blocks of the systems in question and is thus of a more general nature. Our results give insights for the design molecules with a specific…..
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